1. Field of the Invention
This invention relates to a charging member used in an image-forming apparatus, such as an electrophotographic apparatus, and more particularly to a charging member used for a photosensitive-member-charging member, a transfer member, or a developing member and so forth which electrostatically charges the surface of a member to be charged in an image-forming apparatus, such as an electrophotographic apparatus. This invention also relates to an image-forming apparatus, such as an electrophotographic apparatus, and a process cartridge.
2. Related Background Art
In an image-forming apparatus, such as an electrophotographic apparatus (e.g., copying machines and laser printers), and an electrostatic recording apparatus, contact charging is known as a means for charging a member, such as an electrophotographic photosensitive member and a dielectric member. The contact charging is conducted by bringing into proximity to, or contact with, the member to be charged a charging member to which a voltage is kept applied. Compared with corona charging, the contact charging is advantageous, because the voltage applied to the charging member can be lowered and ozone generation can be reduced. The charging member for contact charging must have lower conductivity in order to prevent a leak caused by pinholes, scratches or the like of the surfaces of the member to be charged.
For example, a transfer roller is used in an image-forming apparatus such as an electrophotographic apparatus to transfer a toner image held on a first image-bearing member, such as an electrophotographic photosensitive member, an intermediate transfer member, or a transfer drum, to a second image-bearing member, such as a transfer sheet. The transfer roller brings, via a member to be charged, a transfer sheet into pressure contact with the first image-bearing member holding the toner image on its surface, and electric charges with a polarity opposite to that of a toner constituting the toner image are provided to the transfer roller to electrostatically charge the transfer sheet, whereby the toner is attracted to the paper and the toner image is transferred onto the transfer sheet. In the step of transferring such a toner image, the density of electric charges provided to the transfer sheet has a great influence on the transfer efficiency of the toner image, i.e., image quality.
More specifically, if the transfer sheet has a small electric-charge density, it is weak in toner attraction force. Hence, especially when dry paper is used as the transfer sheet, “scatter” of toner (i.e., spots around line images) may occur. If, on the other hand, it has a large electric-charge density, the toner may be charged to an opposite polarity, and this may cause a “blur” on images transferred, so that images with high-quality cannot be formed on the transfer sheet in some cases. Also, if an entirely black image, i.e. a solid black image, is transferred to the transfer sheet, density non-uniformity may appear or spotlike transfer non-uniformity, such as “sand,” may appear on images transferred. Hence, it is desired for the transfer sheet surface to have uniform electric-charge density.
To meet such a demand, a conductive rubber roller is conventionally proposed which has, on the periphery of a conductive mandrel material such as a metal, a rubber layer in which conductive particles (e.g., carbon black, graphite particles, particles of metal oxides such as titanium oxide and silver oxide, powder of metals such as Cu and Ag, and particles made conductive by coating and treating particle surfaces with any of these) have been mixed and dispersed.
However, in the above conductive rubber roller, it is difficult to control resistance value (electrical resistance) of the rubber layer. Dispersion non-uniformity of the conductive particles in the rubber layer may also make the resistance value locally uneven, so that such unevenness may cause charging non-uniformity or may break the photosensitive member because of a partial leak.
To overcome such disadvantages of the conventional conductive rubber roller, a method is proposed in which a charging roller is obtained using, e.g., a polar polymer such as acrylonitrile butadiene rubber (NBR) or an epichlorohydrin type copolymer. A method is further proposed in which these polar polymers are blended so that the conductivity can be optimized.
For example, Japanese Patent Application Laid-Open No. 11-065269 discloses that a conductive rubber roller improved in resistance uniformity and resistance to ozone is obtainable by using a rubber composition obtained by polymerizing NBR having an acrylonitrile content of 18 to 40% by weight and an epichlorohydrin type copolymer having an ethylene oxide content of 10 to 40 mole %, in a proportion of 80:20 to 20:80 in weight ratio.
Japanese Patent Application Laid-Open No. 2001-115005 discloses a roller made to have a low surface frictional resistance and a low volume resistivity by using only a polyether polymer or a blend composition thereof with an ethylenically unsaturated nitrile-conjugated diene type copolymer rubber.
Japanese Patent Application Laid-Open No. 2002-105305 further discloses a conductive rubber roller or conductive rubber belt used in charging rollers, developing rollers, transfer belts or the like of copying machines and so forth, which makes use of a conductive rubber composition in which an ethylene oxide-propylene oxide-allyl glycidyl ether terpolymer containing ethylene oxide, propylene oxide and allyl glycidyl ether in a copolymerization ratio of 50 to 85 mole %, 1 to 49 mole % and 1 to 10 mole %, respectively, and having a number-average molecular weight (Mn) of 10,000 or more is mixed in epichlorohydrin rubber in a weight ratio of 0.01 or more and 4.00 or less. It is stated that this is effective in achieving low volume resistivity, small compression set and less contamination of photosensitive members.
Japanese Patent Application Laid-Open No. 2002-121376 still further discloses a conductive rubber roller or conductive rubber belt making use of a conductive rubber composition in which (A) an ethylene oxide-propylene oxide-allyl glycidyl ether terpolymer containing ethylene oxide, propylene oxide and allyl glycidyl ether in a copolymerization ratio of 50 to 95 mole %, 1 to 49 mole % and 1 to 10 mole %, respectively, and having a number-average molecular weight (Mn) of 10,000 or more is mixed with:    (B) acrylonitrile butadiene rubber; and    (C) epichlorohydrin rubber;in such a way that, representing the weights of the above (A), (B) and (C) by a, b and c, respectively, the following relationship is established:0.01≦b/(a+b+c)≦0.75; and0.01≦a/c≦2.00.
Then, the present inventors studied charging members making use of conductive rubber compositions in which various polar polymers according to the above prior art are blended. As a result, they have reached a finding that, in particular, the charging member composed of the conductive rubber composition containing the components (A) to (C) disclosed in the above Japanese Patent Application Laid-Open No. 2002-121376 can provide a preferable charging member having a low and uniform resistance value. However, further research on this charging member revealed that the resistance value of the charging member may vary greatly from its initial value when the charging member is electrified for a long time; hence, the resistance variations of the charging member when used for a long term must be kept small in order to provide high-quality electrophotographic images stably over a long period of time.